This disclosure relates to the measurement of total pressure of a flowing fluid.
The total (i.e., stagnation) pressure of a flowing fluid can be used in combination with a measured static fluid pressure to determine a dynamic fluid pressure, which can be used to provide the flow velocity of the fluid. For contained flowing fluids flowing through a conduit or channel of known size, the flow velocity can be converted to a volumetric flow rate and, if the fluid density is known, a mass flow rate. For uncontained fluids, total pressure is often measured to determine the velocity of an object such as an aircraft or boat through a fluid such as air or water. Total pressure is known to be measured with any of a number of types of Pitot tubes. Pitot tubes typically have an open end in the direction from which the fluid is flowing and are closed on the opposite end. Dynamic pressure from the flowing fluid tends to direct fluid flow into the open end of the tube. At the opposite end, however, there is no outlet from which the fluid can escape so it stagnates. The pressure of stagnant fluid, i.e., stagnation pressure, in the tube is also known as total pressure. The static pressure of the fluid can be measured from a layer of fluid immediately adjacent to a surface that is oriented parallel with the fluid flow direction, such as through a port on an exterior surface of the pitot tube housing or on some other surface in contact with the flowing fluid remote from the pitot tube (e.g., an aircraft surface or a flow channel conduit wall).
One problem with Pitot tubes is that their closed end can make them susceptible to the accumulation of foreign solid or liquid particulates entrained in the flowing fluid. Such particulates can plug pressure sensing ports inside the Pitot tube, which can lead to false pressure readings. This is a particularly significant issue for aircraft speed sensors, not only because of the danger of flying without an accurate airspeed reading, but also because aircraft operating conditions such as airborne particulate such as due to clouds, both super-cooled liquid water and ice crystals, or sand/dust and volcanic ash and exposure to ground-based particulates can present unique challenges for the measurement of aircraft airspeed.
A commonly-used approach to the formation of ice particulates in airspeed sensor Pitot tubes is the use of heating elements in combination with the placement of one or more drain holes. These small drains are typically sized and positioned so as to not impact the total pressure at the measurement location inside the Pitot beyond the ability to account for with calibration. Ice barrier features are also used to either prevent ice formation or to promote its formation in non-sensitive areas. However, drain holes are still susceptible to plugging by solid or viscous liquid particulates, and barrier features only relocate the problem but do not eliminate it.